1. Field of the Invention
The present invention relates to a method of selecting substrates for photomask blanks from which photomasks can be fabricated for use in photolithography associated with the manufacture of such products as semiconductor devices.
2. Prior Art
As semiconductor devices continue to become more highly integrated, there is a growing desire for smaller geometries in photolithography. Already, the design rule for devices has been lowered to 0.11 μm and patterns are required to be accurate to 10 nm or less. These trends have led to a number of technical challenges concerning the accuracy of photolithographic processes employed in semiconductor manufacturing.
One such challenge relates to the flatness of the photomask used in lithography. Photomask flatness is an important factor for achieving higher accuracy in the patterning step. As the minimum feature size becomes smaller, the focal depth allowed by the resist becomes smaller, making it impossible to disregard the flatness of the substrate as a cause of focal shift.
To reduce focal shift in lithography, the mask pattern serving as the original must be positioned with sufficient precision to enable the exposure pattern to be written at a given position and linewidth within the allowable error during wafer exposure. Hence, a deposited film, such as a patterned masking film or phase shift film, should ideally be formed on a substrate of exceptional flatness. However, when a photomask is mounted in a wafer exposure system, holding the photomask on the mask stage of the exposure system with a vacuum chuck in the manner disclosed in JP-A 2003-50458 can greatly deform the overall surface shape of the photomask, depending on the surface shape of the clamped portion of the photomask.
The photomask is generally fabricated by depositing on a transparent substrate a film such as a masking film, a semi-transparent film or a phase shift film, or a combination thereof, to produce a photomask blank. A resist is then applied onto the photomask blank to form a resist film, which is lithographically processed. Using the processed resist film as the etching mask, a mask pattern is formed. The resist film is finally stripped off.
A photomask is thus obtained after passing through a long series of operations. The transparent substrate used as the starting material in this process is prepared by cutting a slice from an ingot of synthetic quartz, and polishing the surfaces of the slice. Until now, this surface polishing operation has been carried out only with the intention of reducing surface defects or minimizing warpage of the substrate surface so as to provide a flat surface. Therefore, nothing has previously been known about how to modify the surface shape of the photomask substrate to achieve an even smaller minimum feature size in a pattern written on a wafer substrate; that is, specifically what shape the surface of the photomask should be given where it will be held with a vacuum chuck, based on the shape the photomask assumes when held by the vacuum chuck. Nor has anything been known about what sort of method may be used to form such a surface shape in order to enable the production of substrates having the desired surface shape in a good yield or about how to suppress the formation of defects on the substrate itself or of photomask defects that originate from the substrate.